Methionine is the only naturally occurring amino acid known to initiate protein synthesis (Lucas-Lenard, J. and F. Lipmann, Ann. Rev. Biochem., 40:409-448 (1971)). In prokaryotes, the amino acid is first attached to the initiator methionine tRNA by methionyl-tRNA synthetase and subsequently formylated by methionyl-tRNA transformylase. The resulting f-Met-tRNA.sup.fMet initiates protein synthesis in a reaction dependent on initiation factor 2 (IF-2).
Aminoacylation of E. coli methionine tRNAs depends on recognition of the methionine anticodon CAU by E. coli methionyl-tRNA synthetase (Schulman, L. H. and H. Pelka, Proc. Natl. Acad. Sci., USA, 80:6755-6759 (1983)). Mutations in this sequence lead to loss of methionine acceptor activity and, in some cases, to acquisition of a new amino acid acceptor activity corresponding to that of the altered anticodon sequence (Schulman, L. H. and H. Pelka, Biochemistry, 24:7309-7314 (1985); Schulman, L. H. and H. Pelka, Science, 242:765-768 (1988); Schulman, L. H. and H. Pelka, Science, 246:1595-1597 (1989); Schulman L. H. and H. Pelka, Nucleic Acids Res., in press (1990).